sc handler optimisations

[andymck]

Provides optimisations/improvements of the state channel GRPC handler and the shared blockchain_state_channel_common module. This mod is shared across both grpc and libp2p transport layers for state channel requests.

Optimisation/improvements include:

• Remove cached versions of the blockchain and ledger in the state channel handler state
• Add a simple ETS cache to blockchain_worker to hold the current blockchain. Avoids blocking calls to the worker by spawned grpc handlers.

[andymck]

So the approach I took was to insert to the cache anytime the chain in state is being updated